1. Field of the Invention
The present invention relates to a wavelength-tunable light source.
2. Related Background Art
Many molecules display strong absorption in a mid-infrared range (at a wavelength of 2 μm or more), and this wavelength range is known as a fingerprint region. In recent years in particular, the interest in environmental measurement has been increasing, and concentrations of various greenhouse gases have been measured on the basis of absorption in the mid-infrared range. Further, there is a range with a high atmospheric transmittance in a wavelength range of 2 μm to 5 μm, and this wavelength range is called the atmospheric window.
Quantum cascade lasers have been researched and developed as mid-infrared light sources utilizing electronic intersubband transitions, which enables environmental measurement and bioinstrumentation by making use of the feature of the mid-infrared range as described above. The light-emitting properties of general semiconductor lasers are determined by solid state properties of their materials. On the other hand, the light-emitting properties of quantum cascade lasers can be artificially designed and controlled. Accordingly, a quantum cascade laser may be ingeniously designed to be capable of simultaneously emitting light at a plurality of wavelengths, and of having gains in a wide band with a bandwidth ranging several μm (from 2 μm to 20 μm for example) in the mid-infrared range.
Combining such a quantum cascade laser having gains in a wide band with an external resonator may realize a compact wavelength-tunable light source which is capable of performing wavelength tuning in a wide band (see EP Patent Application No. 2081265 and International Publication No. 2008/036884). Using such a single wavelength-tunable light source allows application for a variety of uses.
In a wavelength-tunable light source in which a quantum cascade laser and an external resonator are combined, a light emitted from a first end of the quantum cascade laser is collimated by an optical system to be thereafter made incident on its diffractive grating, and a light at a particular wavelength in the light made incident on the diffractive grating is diffracted in a direction opposite to the incident direction to be returned to the first end of the quantum cascade laser. The light emitted from a second end of the quantum cascade laser is made incident on a reflecting mirror composing the external resonator with the diffractive grating, and the light is partially reflected by the reflecting mirror and the remaining light transmits through the reflecting mirror. The light transmitting through the reflecting mirror is to be an output light from the wavelength-tunable light source. The wavelength of the light returned to the first end of the quantum cascade laser from the diffractive grating (that is, an output light from the wavelength-tunable light source) can be adjusted by adjusting a direction of the lattice plane of the diffractive grating. Such an external resonator is known as one having a Littrow configuration.